Created attachment 132909 [details]
HTML mail with breeze Dark

SUMMARY
Many if not all HTML mail composers assumes that the background is light, and thus doesn't specify a background colour. If you use e.g. Breeze dark it often leads to black letters on dark grey background.

STEPS TO REPRODUCE
1. Use Breeze Dark theme.
2. Go through your HTML emails
3. I betcha at least some shows the issue as shown in the attachment.

OBSERVED RESULT
Almost unreadable emails

EXPECTED RESULT
Readable emails ;)

SOFTWARE/OS VERSIONS
Operating System: Kubuntu 19.10
KDE Plasma Version: 5.18.3
KDE Frameworks Version: 5.67.0
Qt Version: 5.12.4
Kernel Version: 5.3.0-64-generic
OS Type: 64-bit
Processors: 2 × Intel® Pentium® CPU B960 @ 2.20GHz
Memory: 7,7 GiB of RAM

ADDITIONAL INFORMATION
Nextcloud mail had the same problem, but apparently it has been solved by somehow changing the theme. Here it could possibly be solved by having a light background in HTML emails perhaps with an option in Settings/Appearance/Colors.
https://github.com/nextcloud/mail/issues/1742#issuecomment-530347369